The invention relates to a microprocessor-controlled apparatus including an anti-theft protection means which can be disabled. More particularly, the invention relates to a consumer apparatus which is considered liable to theft. Such an apparatus may be, for example, a car radio, CD player, video recorder, camera recorder, and the like.
An apparatus of the type described in the opening paragraph is known from Netherlands Patent Application NL 8304138. A control circuit in this known apparatus is adapted, inter alia, to perform a legitimation program for putting the apparatus into operation after it has been connected to the supply voltage. The legitimation program comprises a comparison of a legitimation code, entered by the user, with a security code stored in a first memory. Unauthorized use of the apparatus is thus prevented, rendering theft unattractive.
In practice it has been found useful to offer the owner of an apparatus the facility of disabling the anti-theft protection means. This is notably the case when using a car radio which is mounted in a so-called slide and is each time taken along by the owner in order to prevent car damage caused by burglars. To this end the apparatus comprises a second memory for storing a security variable which is constituted in the known apparatus by an indication bit indicating whether the protection means is enabled. If the protection means is not enabled, the legitimation program is not performed so that the car radio can be readily put into operation after it has been placed in the slide. Said second memory for storing the security variable is necessarily a non-volatile RAM. In practice it is constituted by an external memory coupled to the microprocessor. The known apparatus has the drawback that such an external memory is externally accessible to frauds. For example, the indication bit of a protected car radio can be read with the aid of auxiliary means and stored again after inversion. The car radio is then no longer protected and is therefore attractive to the illegal trade.